Harden DisplayItemList in release mode not to crash on cached display item failures.

Source/platform/graphics/paint/DisplayItemList.h

Index: Source/platform/graphics/paint/DisplayItemList.h
diff --git a/Source/platform/graphics/paint/DisplayItemList.h b/Source/platform/graphics/paint/DisplayItemList.h
index 51dc0f8289d6a6b88eccc3d471a86f9de37706bb..979f81002ff5d1fda9ff22ea4790327f26098862 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -89,8 +89,8 @@ private:
 
     static size_t findMatchingItemFromIndex(const DisplayItem::Id&, DisplayItem::Type matchingType, const DisplayItemIndicesByClientMap&, const DisplayItems&);
     static void addItemToIndex(DisplayItemClient, DisplayItem::Type, size_t index, DisplayItemIndicesByClientMap&);
-    DisplayItems::Iterator findOutOfOrderCachedItem(DisplayItems::Iterator& currentIt, const DisplayItem::Id&, DisplayItem::Type, DisplayItemIndicesByClientMap&);
-    DisplayItems::Iterator findOutOfOrderCachedItemForward(DisplayItems::Iterator& currentIt, const DisplayItem::Id&, DisplayItem::Type, DisplayItemIndicesByClientMap&);
+    DisplayItems::Iterator findOutOfOrderCachedItem(DisplayItems::Iterator currentIt, const DisplayItem::Id&, DisplayItem::Type, DisplayItemIndicesByClientMap&);
+    DisplayItems::Iterator findOutOfOrderCachedItemForward(DisplayItems::Iterator currentIt, const DisplayItem::Id&, DisplayItem::Type, DisplayItemIndicesByClientMap&);
 
 #if ENABLE(ASSERT)
     // The following two methods are for checking under-invalidations